Shell ovate, lenticular, compressed, with two short divergent spines on the apical margin (a dorsal and a ventral). Peristome slender, semitubular, half as long as the radius, with three terminal teeth, two short, vertically ascending, paired teeth and a larger odd tooth, which is inclined obliquely over the mouth.
Dimensions.—Diameter of the shell 0.15 to 0.2, peristome 0.05 to 0.07.
Habitat.—North Atlantic, Færöe Channel, Gulf Stream, surface and at various depths, John Murray.
9. Challengeron richardsii, n. sp. (Pl. [99], fig. 8).
Shell subcircular, lenticular, strongly compressed, with two stout, divergent, straight spines on the aboral margin (a dorsal and a ventral) which reach the length of the radius. Peristome slender, semitubular, slightly curved, about as long as the radius, with three short, divergent, terminal teeth. Mouth obliquely truncate. The dorsal odd tooth is nearly vertical, the two lateral paired teeth almost horizontal.
Dimensions.—Diameter of the shell 0.2 to 0.25, length of the spines and the peristome 0.08 to 0.12.
Habitat.—Antarctic Ocean, Station 157, surface, common.
10. Challengeron fergusoni, n. sp. (Pl. [99], fig. 9).
Shell isosceles triangular, with three slightly convex edges and two short, nearly vertically descending spines on the two corners of the aboral margin. Peristome inversely funnel-shaped, with a deep ventral incision on the base, and three broad, ovate, triangular teeth. The dorsal odd tooth is nearly vertical, whilst the two lateral paired teeth are almost horizontal; therefore the angle between the former and the latter is about a right angle.
Dimensions.—Diameter of the shell 0.2 to 0.3, length of the peristome 0.1.